Dual bit drilling tool

ABSTRACT

A dual bit head drilling mechanism for use on the end of a conventional drill string for drilling through all types of subterranean structure, both hard and soft, without requiring pulling of the drill string to change bits whenever a different type or hardness of rock is encountered. The mechanism includes a two-pronged drag bit using tungsten carbide inserts and a surface set diamond bit. The two bits can be alternately brought into use merely by increasing or decreasing the amount of thrust on the bit head mechanism. A cable and sheave assembly interconnects the driving sleeve, tungsten carbide bit head and diamond bit head, and compressible Belleville washer springs are provided for normally biasing the diamond drill bit head into drilling operation.

[ Oct. 16, 1973 Primary ExaminerDavid H. Brown Att0rney-Ralph F.Crandell et al.

[57] ABSTRACT A dual bit head drilling mechanism for use on the end of aconventional drill string for drilling through all types of subterraneanstructure, both hard and soft, without requiring pulling of the drillstring to change bits whenever a different type or hardness of rock isencountered. The mechanism includes a two-pronged drag bit usingtungsten carbide inserts and a surface set diamond bit. The two bits canbe alternately brought into use merely by increasing or decreasing theamount of thrust on the bit head mechanism. A cable and sheave assemblyinterconnects the driving sleeve, tungsten carbide bit head and diamondbit head; and compressible Belleville washer springs are provided for'normally biasing the diamond drill bit head into drilling operation.

Irvin P. Nielsen, 1315 S. Clarkson St., Denver, both of Colo.

Dec. 1, 1971 Inventors: Edward C. Rosar, 13352 W.

Dakata Ave., Lakewood;

Filed:

Appl. No.: 203,536

US. 175/319, 175/329, l75/38l Int. E2lb 9/00, E2lb 9/36 FieldofSearch...........175/381-383, 319,

References Cited UNITED STATES PATENTS United States Patent Rosar et al.

[ DUAL BIT DRILLING TOOL 175/330 l75/38l X r 14 Claims, 9 DrawingFigures Sorensen.........................

Henderson...... Hildebrandt l llldebrandt PATENTED OCT 16 I975 SHEET 30F4 SHEET 0F 4 JPMENTEDUBT 16 I975 FIG? DUAL BIT DRILLING TOOL BACKGROUNDOF THE INVENTION This invention relates to drilling tools and moreparticularly to drilling tools of the multiple bit type. When referenceis made to drilling in the present application, it will be understoodthat rotary drilling is intended and is distinguished from percussion orimpact drilling.

In many, if not most, drilling operations in which it is desired todrill deep holes through earth and rock materials, there is normallyencountered a wide variety of subterranean strata, the structures ofwhich vary widely and include both hard and soft rock and earthformations. Conventional practice requires that a particular type of bitbe utilized depending upon the structure being drilled. The type ofrotary drilling tool utilized depends primarily upon the structure beingdrilled. Two widely known types of rotary drilling tools are thetungsten carbide drag bit, which includes a pair of drilling bladeshaving tungsten carbide inserts secured to their drilling edges. Thistype of tool is conventionally used for relatively soft structures.Where hard, dense structures are encountered, the conventional practiceis to utilize a diamond drill bit which involves a bit head with anumber of diamond studs inset into its drilling surface.

Conventional drilling practice requires that the bit be changed when achange in subterranean structure is encountered. For example, if atungsten carbide bit is being used and a hard strata is encountered, itis necessary to pull the entire drill string, change the bit to adiamond drill bit, and then lower the string back into the hole.Obviously, the deeper the hole the more difficult and time consuming isthe process of changing drill bits. Much drilling time is lost inpulling the string and changing the bit, thus substantially increasingthe cost of the drilling operation.

Because the diamond inserts are extremely hard, but

relatively fragile, the diamond drilling operation must be carried outunder lower drilling forces than are utilized in drilling with tungstencarbide bits. The diamond bit depends primarily upon a wearing orgrinding action under relatively low thrusts, whereas the tungstencarbide drag bits involve a digging type of action under relativelyhigher thrusts. If too large a force is used on the diamond drill bits,the diamonds can be shattered. On the other hand, it is desired to usemuch higher drilling forces and thrusts in order to obtain adequatedrilling action with the tungsten carbide bits.

V In the prior art there has been suggested the possibility of usingdual bits. In U.S. Pat. No. 1,748,341 issued Feb. 25, I930 to J. Grantet al. there is disclosed a Rotary Rock Bit which includes rotarydrilling teeth in combination with percussion teeth. When the drillingteeth meet a strata which increases the resistance to rotation, the toolrotates and actuates the percussion teeth to break through theparticular strata causing the resistance.

In U.S. Pat. No. 2,819,043 issued Jan. 7, 1958 to H. 1. Henderson for aCombination Drilling Bit, there is disclosed a combination of a tungstencarbide bit and a diamond bit. The Henderson structure is so constructedthat the tungsten carbide bits are operative under relatively lowdrilling pressures. These bits are spring mounted so that when drillingin soft formation they will spring downwardly and slightly imbed thecutting edge into the soft formation. If, while drilling in a softformation, the tungsten carbide bit drills into a hard formation, then,upon an increase in drilling pressure, the spring blades on which thetungsten carbide bits are mounted will be deflected to allow the diamonddrill bit to bear on the hard formation. The drill structure asdisclosed by Henderson operates contrary to accepted practice in that itrequires relatively high forces for the diamond drilling bit to beutilized and relatively low forces for the tungsten carbide bit to beutilized. Also, it will be noted that the tungsten carbide bit bladescontinue to ride on the bore hole end even while the diamond bit isdrilling.

OBJECTS OF THE PRESENT INVENTION It is the principal object of thepresent invention to provide a new and improved drilltool for use on theend of a conventional drill string for drilling through all types ofsubterranean structures, both hard and soft, without requiring thepulling of the drill string to change bits whenever a different type orhardness of rock is encountered.

More particularly, it is an object of the present invention to provide adrill tool of the above character which utilizes both a tungsten carbidedrag bit and a diamond insert drill bit. More specifically, it is anobject of the invention to provide a dual bit structure of the foregoingcharacter in which the diamond insert drill bit is positioned indrilling operation under relatively low drilling thrusts while thetungsten carbide drag bit is positioned for drilling operation underrelatively higher drill thrusts, and the drilling bit not in use ispositively retracted from drilling engagement with the end of the borehole.

r Another object of the present invention is to provide a drill tool ofthe foregoing character which affords time and cost savings associatedwith not having to pull a drill string to change bits.

A further object of the invention is to provide a drill tool of theabove character which is simple yet rugged in construction, requires aminimum of maintenance under severe drilling conditions, and may beutilized in place of conventional single purpose drilling tools.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal elevation view,partly in section and with parts cut away for clarity, of a dual bitdrilling tool embodying the present invention, with the diamond drillbit extended into drilling position and the tungsten carbide drill bitretracted.

FIG. 2 is a view similar to FIG. 1 but showing the tungsten carbide dragbit in drilling position and the diamond drill bit retracted.

FIG. 3 is a front or drilling end view of the drilling tool shown inFIG. 1.

FIG. 4 is a section view taken substantially in the plane of line 44 onFIG. 1.

FIG. 5 is a section view taken substantially in the plane of line 55 onFIG. 1.

FIG. 6 is a section view taken substantially in the plane of line 6-6 onFIG. 1.

FIG. 7 is a longitudinal section taken through a drilling tool embodyinga modified form of the present invention and showing the diamonddrilling bit in drilling position.

FIG. 8 is a longitudinal section view similar to FIG. 7 and showing thetungsten carbide bits in drilling position.

SUMMARY The invention, in accordance with the objects outlined above,resides in a dual bit head drilling mechanism designed for use on theend ofa conventional drill string for drilling through all types ofsubterranean structure, both hard and soft, without requiring pulling ofthe drill string to change bits whenever a different type or hardness ofrock is encountered. The mechanism embodies two bit heads, a two-prongeddrag bit using tungsten carbide inserts, and a butterfly" shaped surfaceset diamond bit head.- Either of the two bits can be brought into use byselectively increasing or decreasing the amount of thrust on the bithead mechanism. lf, while drilling through hard rock using the diamonddrill bit head, the drill penetrates into a softer structure where thepenetration rate could be increased by using a drag bit, the operatorsimply increases the drill pipe thrust above a level predetermined bythemechanism design. This action withdraws the diamond bit head and extendsthe tungsten carbide bit head to do the cutting. If hard rock is againencountered, the operator decreases the drill pipe thrust until it isbelow the predetermined level, which action automatically extends thediamond bit into position for drilling the hard rock.

The bit mechanism is housed in a driving sleeve which is secured to theend of a drill pipe string. The tungsten carbide bit head is driven by aspline connection with the driving sleeve, while the diamond drill bithead nests between the two tungsten carbide bit prongs and is driven bya spline connection with the tungsten carbide bit head. A sheave andcable assembly, including sheaves mounted on the tungsten carbide bithead and cables passing over the sheaves and connected at one end to thediamond drill bit head and at the other end to the driving sleeve,provide the interconnecting arrangement between the bit heads. Housedbetween the diamond drill bit head and the tungsten carbide bit head area series of spring or Belleville washers which normally urge the diamonddrill bit head-into drilling position. When the washers are compressed,by increasing the thrust on the drill bit housing, the cable and sheaveassembly serves to retract the diamond drill bit head inwardly ortelescopingly into the tungsten carbide bit head, and simultaneouslyextend the tungsten drag bit into drilling position by compressing thesprings. Coolant passages are provided through the drill bit headstructure to allow the flow of coolant and lubricatingfluids.

An alternative mechanism is described for interconnecting the drivingsleeve, tungsten carbide bit head and diamond drill bit head, includinggears mounted on the tungsten carbide bit structuremeshing with racks onthe driving sleeve and diamond 'bit head. Various types of resilientmechanisms such as coil springs, Belleville springs, or othercompressible resilient materials may be utilized.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, thereis indicated, by the general reference numeral 10, a drilling toolembodying the present invention and secured by a conventional taperedthread 11 to the end of a drill pipe string 12. The drill pipe string 12forms a part of a conventional drilling rig and is adapted to be raisedand lowered by an appropriate hoist mechanism (not shown), and rotated,to produce a rotary drilling action, by a conventional drill piperotating mechanism (not shown). The drill pipe string 12 is formed insections,

- jointed together by threaded drill pipe connectors, and

for many drilling operations the drill pipe 12 may be composed of anumber of such sections and be many feet in length, depending upon thedepth of the hole to be drilled.

The tool 10 is a dual bit drilling mechanism formed by a housing or tooljoint 14, mounting a tungsten carbide drag bit 15, and a diamond drillbit 16. As discussed above, the tungsten carbide drag bit 15 is adaptedfor drilling through relatively softer strata by the application ofrelatively higher drilling thrusts on the drill pipe, while the diamonddrill bit 16 is adapted for drilling in hard strata but with theapplication of lower drilling thrusts. The tungsten carbide bit head isa two prong drag bit of conventional design utilizing heavy tungstencarbide inserts .18 to do the cutting. The inserts are backed up byheavy members extending down from a flat face and are braised or weldedto these extensions. The diamond drill bit 16 is formed with a head 21which is, in plan view as seen in FIG. 3, of a butterfly shape, the faceand peripheral surfaces of which are studded with a plurality of surfaceset diamond studs 22.

The housing or tool joint 14, tungsten carbide drag bit 15 and diamonddrill bit 16 are operatively associated and interconnected by structuralmeans which position the tool bits relative to each other in such amanner that they can be alternately brought into use merely byincreasing or decreasing the amount of thrust on the drilling tool. Inthe construction shown, the diamond drill bit 16 is normally extendedinto drilling position and the tungsten carbide bit 15 is normallyretracted (FIG. 1). Thus, assuming that the first strata to be drilledis hard, the diamond drill bit 16 is utilized. If, while drillingthrough the hard rock using the diamond bit 16, the drill penetratesinto a soft structure where the penetration rate could be increased byusing the tungsten carbide drag bit 15, the operator simply increasesthe drill pipe thrust above a level predetermined by the mechanicaldesign of the operatively interconnecting means in the tool. This actionwithdraws the diamond bit 16 and extends the tungsten carbide bit 15 todo the cutting (FIG. 2)..If hard rock is again encountered, the operatordecreases the drill pipe thrust until it is below the predeterminedlevel, which action automatically extends the diamond bit 16 permittingit to cut the hard rock, and simultaneously and positively retracts thetungsten carbide bit 15 so that it does not drag or wear on the hardrock.

For rotatively driving the drill bits and providing an operativepositioning connection therebetween, as well as for securing the tool 10to the drill pipe 12, the housing or tool joint 14 is formed as agenerally sleeveshaped two-part member. The housing or tool joint sleeve14 houses the bit assembly and is slidingly and drivingly connectedtherewith to rotate the bits as the drill pipe string 12 is rotated. Tothis end, the housing or tooljoint sleeve 14 is formed with an upperjoint section 24 which is sleeve-shaped in configuration and embodies,atone end, a tapered thread 11 for securing the tool to the drill pipestring 12. At its opposite end, the upper. tool joint section 24 isinternally threaded with a tapered thread 25 which engages the upper endof a lower tool joint section 26. For engaging with the internal threads25 of the upper tool joint section 24, the lower tool joint section 26is externally threaded with a tapered thread 28. For convenience indescription, as hereinafter used the terms upper, rearward, inner ordrill pipe end of the tool will be considered as referring to that endwhich is secured to the drill pipe string 12, and is the right-hand endof the tool as shown in FIGS. 1 and 2. The lower, forward, outer ordrilling end of the tool will be denominated as the drilling end and isthe left-hand portions of the tool illustrated in FIGS. 1 and The lowertool joint section 26 may, for convenience in fabrication, be formed intwo parts 26a and 26b which are welded or otherwise secured together byany suitable means, such as the weld illustrated at 29. The lower tooljoint section part 26a is generally sleeveshaped in cross section andtelescopingly receives the tungsten carbide drag bit 15. For providingrotary driving power to the tungsten carbide bit 15, as well as forpermitting the tungsten carbide bit to slide or telescope axially withinthe tool joint 14, the lower tool joint part 260 is provided, at a pointadjacent its forward end, with internal splines 30. These splinesslidingly and drivingly engage with corresponding external splines 31 onthe tungsten carbide bit 15.

The tungsten carbide drag bit is telescopingly inserted for slidingmovement within the tool joint housing 14, and the splines 30 on thehousing sleeve 26a drivingly engage spline 31 on the tungsten carbidebit. To this end the tungsten carbide drag bit 15 includes a tubular orsleeve portion 32 threaded with external threads 34 at its rear or innerend, and formed at its outer end with a two-pronged head 35. The bithead 35 is also tubular, having a bore 36 therethrough which is smallerin diameter than the bore 38through the sleeve portion 32 of thetungsten carbide bit, thereby defining an internal shoulder 39 withinthe bit. The head 35 includes two prongs or blades 40, on which thetungsten carbide inserts 18 and their backing portions are mounted. Therear or upper end of the bit sleeve 32 is engaged, by means of theexternal threads 34, with a tungsten carbide bit head closure assembly41.

As indicated above, the diamond drill bit head 16 is butterfly" shaped,and nests between the two tungsten carbide inserts 18, as shown in FIG.3. The diamond drill bit head 21 is secured to a diamond drill bit shaft42, which is telescopingly received within the bore 36, 38 through thesleeve portion 32 and head 35 of the tungsten carbide bit 15. The head21 is secured to the shaft 42 by means of a bolt 44, and a recess 45isprovided in the tungsten carbide bit head 35 for receiving the bolt 44when the diamond drill bit head 21 and shaft is in the retractedposition. The. shaft 42 further includes an external flange 46 whichprovides a forward shoulder 48 in opposition to the tungsten carbide bithead shoulder 39. As shown in FIGS. 1 and 3, the butterfly-shapeddiamond drill bit head 2l is provided with drive lugs 43 which areslidably and drivingly engaged with corresponding external grooves 47 inthe tungsten carbide bit head 35. The coaction of the lugs 43 and thegrooves 47 operatively couples the diamond drill bit 16 to the tungstencarbide bit 15 for rotary driving movement thereby.

For biasing the diamond drill bit 16 and shaft 42 axially outwardly withrespect to the tungsten carbide bit 15 and two-pronged bit head 35,resilient means 49 are mounted in the space defined between the diamonddrill bit flange 46 and the tungsten carbide bit closure 41. Anyappropriate means may be used, such as a coil spring or othercompressible resilient material or structure. In the present instance,it has been found that a stack of Belleville spring washers 49 affordsthe desired spring action between the tungsten carbide drill bit 15 andthe diamond drill bit 16. Belleville washers are conical spring washerswhich, when acted upon by an axial force, will flatten, and when theaxial force is removed, will spring back to their conical shape. Eachwasher requires a certain force to flatten it, and when stacked as shownin FIGS. 1 and 2 of the drawings, it is possible to obtain a very highspring force with a small deflection, that is a high spring rate, in alimited space. By varying the number of spring washers in the stack, theaxial spring force can be varied, and hence the drilling thrust requiredto position the tungsten carbide drag bit into drilling position can bepredetermined. The limit of axial movement of the diamond drill bit 16axially: outwardly with respect to the tungsten carbide bit 15 isdetermined by the engagement of the shoulder 39 with the tungsten bithead 35 and the forward flange shoulder 48 of the flange 46 on thediamond drill bit shaft 42. The Belleville spring stack 49 is compressedin place between the diamond drill bit shaft flange 46 and a shoulder 50on the tungsten carbide bit head closure 41.

With the spring-structure as shown, the diamond drill bit 16 will benormally positioned in drilling position and the tungsten carbide bithead 35 will be retracted into the tooljoint 14 (FIG. 1). Forpositioning the tungsten carbide drag bit in drilling position, as afunction of the thrust exerted by the drill pipe string 12 on thetooljoint housing 14, the tool joint 14, tungsten carbide bit 15 anddiamond drill bit 16 are mechanically interconnected by a mechanismwhich retracts the diamond drill bit shaft 42 and correspondinglyextends the tungsten carbide bit relative to the diamond drill bit whenthe thrust on the tool joint 14 exceeds a thrust value as predeterminedby the spring force exerted by the Belleville spring stack 49. To thisend, the tungsten carbide bit head closure 41 is slotted with four slots52 to provide four sets of pairs of mounting prongs 54, 55. Between eachset of prongs 54, 55, there is rotatably mounted a cable sheave orpulley 56, each sheave being rotatably mounted on a pin 58 extendingbetween each set of mounting prongs 54, 55. Four cables 59 are provided,each of which is secured at one end to a shaft cap cable mount 60threadably engaged with the rear or inner end of the diamond drill bitshaft 42,

. passes over an associated sheave 56 and is secured at its other end tothe end of the lower tool joint section 26. Adjustable cable anchors 61are provided securing each cable 59 to the shaft cap 60, and cableanchors 62 are provided for securing the other end of the cable 59 tothe tool joint section 26.

With the foregoing structure, it can be readily observed that a downwardthrust on the tool joint 14, in excess of a value determined by thespring rate of the Belleville stack 49, will retract the diamond drillbit and extend the tungsten carbide drag bits as shown in FIG. 2. On theother hand, a thrust on the tool joint 14 which is less than the thrustvalue determined by the spring rate of the Belleville stack, will permitthe spring stack 49 to extend the diamond drill bit into drillingposition and consequently retract the tungsten carbide bit 15. Theforces acting in the system are shown in the diagrammatic representationof the system illustrated as FIG. 9 in the drawings. As can be seen withthis arrangement, a downward thrust W on the drill pipe is transferred,through tension in the cables 59, to exert an equal upward force on thediamond drill bit shaft 42. By virtue of the tension in the cables 49and their 180 wrap over the sheaves 56, this thrust force W is two andtransferred to the tungsten carbide bit head 35. So long as the thrustforce W is equal to or less than the spring force,'indicated in thedrawing by the reference character F the system will remain in balancewith the thrust W reacted directly by the diamond bit against the bottomof the hole which is being drilled. As the thrust is increased, however,the downward force on the tungsten carbide bit head 15 exceeds thespring force F thereby compressing the stack of washers 49. This actionextends the tungsten carbide bit 15 below the face of the diamond bit16, transferring the drill pipe thrust W directly to the tungstencarbide bit. Any further increase in the drill pipe thrust beyond thatrequired to flatten the washers will be reacted directly by the tungstencarbide bit 15.

In the embodiment shown, groups of the Belleville washers arepreferrably alternated in the spring stack 49. This is a matter ofdesign, as is the number of Belleville spring washers actually utilized.The relative movement between the tool joint housing 14 and the tungstencarbide bit 15 is limited by engagement of the splines 26 with ashoulder 64 defined on the tungsten carbide bit head 35, while theforward movement ofthe tungsten carbide bit head is limited by the totalcompressed thickness of the stack of spring washers 40 and engagement ofthe flange 46 with the bit head shoulder 39.

During actual drilling operation it is desirable to circulate drillingfluid, coolant, mud or the like through the tool. Passages for the flowof coolant are provided by slotted holes 65 at the bottom of the sheaveslots 52 in the bit head closure 41. These holes 65 extend radiallyinwardly in line with passages 66 in the diamond drill bit shaft cap 60to allow coolant to flow into and down through a passage 68 extendingaxially through the diamond drill bit shaft 42. From the shaft passage68 the coolant fiows outwardly through two ports 69 opening in thesurface of the diamond bit head 21. To provide coolant to lubricate thesplines 30, 31, passages 70 are provided in the upper or rearward ringof the driving sleeve portion 26b of the tool joint (FIG. From thesplines, the coolant passes through further holes 71 at the lower end ofthe splines to the face of the tungsten carbide bit where it is directedagainst the tungsten carbide inserts 18. Additional cooling ports andpassages may be provided as required to prolong tool life.

MODIFIED EMBODIMENT OF THE INVENTION A modified form of the invention isillustrated in FIGS. 7 and 8. In this modification, the operation of thetool is similar to that described above. A modified form of themechanism interconnecting the tool joint, tungsten carbide drag bit anddiamond drill bit is employed. Essentially, the change resides in theuse of interengaging gear and rack mechanisms in lieu of the sheave andcable interconnecting mechanism described above. The structure is shownin FIGS. 7 and 8 in a somewhat diagrammatic form in order to illustratewith greater clarity the modified embodiment of the invention.

In the modified form of the drilling tool, indicated at in FIGS. 7 and8, there is provided a housing or tool joint 84, telescopingly anddrivingly mounting a tungsten carbide drag bit 85, which in turntelescopingly and drivingly mounts a diamond drill bit 86. The tooljoint is adapted to be secured to the end of a drill pipe string (notshown) and the tool is illustrated at the bottom of a bore hole 87. InFIG. 7 the diamond drill bit 86 is shown in drilling position, while inFIG. 8 the tungsten carbide drag bit is shown in drilling position. Acoil spring 88 is utilized as the compressible resilient structure tonormally position the diamond drill bit 85 into drilling positionrelative to the tungsten carbide drag bit 85 as illustrated in FIG. 7.

For operatively positioning the tungsten carbide drag bit into drillingposition in response to an increase in thrust on the tool joint orhousing 84, the tungsten carbide drag bit structure is provided, at itsupper or inner end, with three bifurcated supports 89, each rotatablysupporting a gear wheel 90. The gear wheels 90 on the tungsten carbidebit structure 85 mesh with corresponding racks 91 on the internalsurface of the tool joint or housing 84, and also with a three-sidedrack 92 extending upwardly from the diamond drill bit structure 86. Itcan be readily observed that, with this mechanism, the coil spring 88will ordinarily position the diamond drill bit 85 in drilling position.Upon an increase in thrust on the tool joint housing 84 in excess of theforce determined by the spring rate of the coil spring 88, the tooljoint racks 91 will rotate the gear wheels 89, which will in turn drivethe rack 92 to raise the diamond drill bit 86 against the force of thespring 88 and out of drilling position. The downward force on the tooljoint 84 will simultaneously position the tungsten carbide drag bit 85into drilling position against the bottom of the bore hole 87. Theoperation of and internal forces within the modified structure is thesame as described above and illustrated diagrammatically in FIG. 9.

While certain illustrative embodiments of the present invention havebeen shown in the drawings and described above in considerable detail,there is no intention to limit the invention to the specific formsdisclosed. On the contrary, the intention is to cover all modifications,alternative constructions, equivalents and uses falling within thespirit and scope of the invention as expressed in the appended claims.

We claim as our invention:

1. A drilling tool comprising, in combination, a tool joint housing, atungsten carbide drag bit, a diamond drill bit, and meansinterconnecting said bits and said housing for positioning said tungstencarbide bit in drilling position and positively retracting said diamonddrill bit in response to a thrust on said housing in excess of apredetermined thrust force, and for positioning said diamond drill bitin drilling position and positively retracting said tungsten carbide bitupon the application of a thrust to said tool joint housing below saidpredetermined thrust force.

2. A drilling tool as defined in Claim 1 wherein said tool joint housingcomprises a two part sleeve with an upper section having a threadedportion adapted to be secured to the end of a drill pipe string and alower sleeve section secured to said upper section in axial alignmenttherewith and having internal splines therein alidably and drivinglyengageable with external splines on said tungsten carbide drag bit.

3. A drilling tool as defined in claim 1 wherein said tungsten carbidedrag bit and said diamond drill bit include coolant flow passagestherethrough.

4. A drilling tool comprising, in combination, a tubular tool jointhousing adapted to be secured to the end ofa drill pipe string, a firstdrill bit mounted within said tool joint housing in driving engagementtherewith, a second drill bit mounted within said tool joint housing indriving engagement with said first drill bit, and means operativelyinterconnecting said tool joint housing, said first drill bit and saidsecond drill bit for extending said first drill bit into drillingposition and simultaneously retracting said second drill bit in responseto a downward thrust on said tool joint housing by'said drill pipestring in excess of a predetermined thrust force, and for extending saidsecond drill bit into drilling position and positively retracting saidfirst drill bit in response to a downward thrust on said tool jointhousing by said drill pipe string less than said predetermined thrustforce.

5. A drilling tool comprising, in combination, a tool joint housingadapted to be secured to the end ofa drill pipe string, a first drillbit mounted in said housing for axial telescoping movement with respectthereto and operatively engaged for rotary driving movement by saidhousing, a second drill bit mounted within said first drill bit foraxial telescoping movement with respect thereto and operatively coupledwith said first drill bit for rotary driving movement thereby,compressibly resilient means interposed between said first drill bit andsaid second drill bit for biasing said second drill bit axiallyoutwardly of said first drill bit, and means interconnecting said tooljoint housing, said first drill bit and said second drill bit for movingsaid second bit and said first bit axially with respect to each otherand position said first bit in drilling position in opposition to saidbiasing means upon the exertion of a downward thrust on said tool jointhousing in excess of a force required to compress said biasing means,and for releasing said biasing means to urge said second bit axiallyoutwardly from said first hit upon the exertion of a downward thrust onsaid tool joint housing less than the force exerted by said biasingmeans.

6. A drilling tool as defined in claim 5 wherein said biasing meanscomprises a stack of compressed Belleville spring washers.

7. A drilling tool as defined in claim 5 wherein said interconnectingmeans comprises a plurality of sheaves mounted on said first drill bitand a corresponding number of cables secured at their ends to said tooljoint housing and said second drill bit respectively and passing oversaid sheaves.

8. A drilling tool as defined in claim 5 wherein said interconnectingmeans comprises a plurality of 'gears secured to said first drill bitand racks on said tool joint housing and said drill bit meshing withsaid gears at diametrically opposed points thereon so that said seconddrill bit moves axially oppositely to said tool joint hous- 9. Adrilling tool for selectively drilling relatively harder and softer rockmaterials in response to a change in the downward drilling force appliedto said tool, said tool comprising, in combination. 2.. z nl jointadapted to be secured to the end of a drill pipe and including a drivingsleeve, a first drill bit telescopingly mounted within said drivingsleeve for limited axial movement with respect thereto, cooperativelyengaging means on said driving sleeve and said first drill bit forproviding a sliding and rotary driving connection therebetween, saidfirst drill bit having an axial bore therein, a second drill bit havinga shaft telescopingly mounted within said first bit bore for limitedaxial movement with respect thereto, cooperatively engaging means onsaid first and second drill bits for providing a sliding and rotarydriving connection therebetween, means acting between said first drillbit and said second drill bit for biasing said bits axially apart, andmeans operatively coupling said first drill bit, said second drill bitand said tool joint sleeve for moving said second drill bit and saidfirst drill bit axially with respect to each other and position saidfirst drill bit in drilling position in opposition to said biasing meansupon the exertion of a downward force on said tool joint sleeve greaterthan the force exerted by said biasing means, and for releasing saidbiasing means to urge said second drill bit axially outwardly from saidfirst drill bit into drilling position upon the exertion of a downwardforce on said sleeve less than the force exerted by said biasing meanswhereby said drill bits can be selectively positioned into drillingposition.

10. A drilling tool as defined in claim 9 wherein said biasing meanscomprises a stack of Belleville spring washers.

11. A drilling tool as defined in claim 9 wherein the means operativelycoupling said first bit, said second bit and said tool joint sleevecomprises a plurality of sheaves on said first bit and a plurality ofcables corresponding in number to said sheaves connected at one end tosaid tool joint sleeve and passing over said sheaves and having the'other end connected to said secondbit whereby an increase in thrust onsaid tool joint sleeve in excess of the thrust determined by saidbiasing means retracts said second bit relative to said first bit andextends said first bit into drilling position.

12. A drilling tool adapted to be secured to the end of a drill pipe forselectively drilling both soft and hard rock materials comprising a toolhousing, a tungsten carbide drag bit including a tubular bodytelescopingly and drivingly mounted within said housing, a diamond drillbit telescopingly mounted within said tungsten carbide drag bit body androtatively drivingly engaged with said tungsten carbide drag bit, meansoperatively interposed between said tungsten carbide drag bit and saiddiamond drill bit for biasing said diamond drill bit into a forwardlyextended drilling position relative to said tungsten carbide drag bit,and means operatively interconnecting said tungsten carbide drag bit,said diamond drill bit and said tool housing for retracting said diamonddrill bit relative to said tungsten carbide drag bit in opposition tosaid biasing means in response to an applied drilling force transmittedto said tungsten carbide drag bit by the drill pipe which exceeds theforce of said biasing means and positions said tungsten carbide drag bitin drilling position, said biasing means extending said diamond drillbit into drilling position in bit is positioned in drilling position inresponse to a reduction in drillingforce for drilling relatively hardmaterials.

13. A drilling tool including a tungsten carbide drag bit adapted todrill relatively softer materials upon application of a relativelylarger downward drilling force, and a diamond drill bit adapted to drillrelatively harder materials upon the application of a relatively lesserdrilling force, said tool comprising, in combination:

a tool joint adapted to be secured to the end of a drill a drivingsleeve secured to said joint;

a tungsten carbide drag bit head telescopingly mounted within saiddriving sleeve for limited axial movement with respect thereto andhaving an axial bore extending therethrough;

spline means on said driving sleeve and said head for providing a rotarydriving connection therebetween;

tungsten carbide drill bit inserts secured to the drilling end of saidhead;

a diamond drill bit shaft telescopingly mounted within said drag bithead bore for limited axial movement with respect to said head;

said diamond drill bit secured to the outer drilling end of said shaft;

means on said diamond drill bit slidably engaging said tungsten carbidebit head for providing a rotary driving connection therebetween;

spring means acting between said drag bit head and said diamond drillbit shaft for urging said head and said shaft axially apart;

and means operatively coupling said shaft, said head and said sleeve formoving said head and said shaft axially with respect to each other inopposition to the force of said spring means upon the exertion of adownward force on said sleeve greater than said spring force, and forreleasing said spring to urge said shaft axially outwardly from saidhead upon the exertion of a downward force on said sleeve less than saidspring force;

whereby said tungsten carbide drag bit is positioned to drill relativelysofter materials upon the application of a relatively larger downwarddrilling force on the drill pipe and said diamond drill bit ispositioned to drill relatively harder materials upon the application ofa relatively lesser drilling force on 12 said drillpipe. I j 14. Adrilling tool including a tungsten carbide drag bit and a diamond drillbit comprising, in combination:

a housing generally tubular in configuration and adapted to be securedat one end to a drill pipe;

a driving sleeve secured at one end to said housing and having internalflutes at its other end;

a tungsten carbide drag bit head telescopingly mounted within saiddriving sleeve and having external flutes slidingly and drivinglyengaged with said driving sleeve flutes;

said drag bit head having an axially extending bore therethrough and aninternal shoulder defined in said bore adjacent the outer end of saiddrag bit head; I

tungsten carbide drill bit inserts secured to the outer drilling end ofsaid drag bit head;

a diamond drill bit shaft telescopingly mounted within said drag bithead;

a diamond drill bit head secured to the outer end of said diamond drillbit shaft and drivingly engaged with said drag bit head;

an external flange on said diamond drill bit shaft defining a shoulderthereon, said flange being engagable with said internal shoulder on saiddrag bit head for limiting axial movement of said shaft with respect tosaid drag bit head;

means defining an internal shoulder adjacent the rear end of said dragbit head;

spring means intermediate said diamond bit shaft and said drag bit forurging said shaft forwardly and outwardly of said drag bit;

and means operatively connecting said shaft, drag bit and driving sleeveadjacent the tool joint ends thereof for retracting said shaft againstthe force of said spring upon movement of said driving sleeve forwardlywith respect to said drag bit head to position said tungsten carbide bitin drilling position upon an increase in force upon the drill pipe, andfor retracting said tungsten carbide drag bit head and enabling saidspring to extend said diamond drill bit shaft to position said diamonddrill bit in drilling position and simultaneously retract said tungstencarbide drag bit head upon a relaxation of pressure upon said drillpipe.

1. A drilling tool comprising, in combination, a tool joint housing, atungsten carbide drag bit, a diamond drill bit, and meansinterconnecting said bits and said housing for positioning said tungstencarbide bit in drilling position and positively retracting said diamonddrill bit in response to a thrust on said housing in excess of apredetermined thrust force, and for positioning said diamond drill bitin drilling position and positively retracting said tungsten carbide bitupon the application of a thrust to said tool joint housing below saidpredetermined thrust force.
 2. A drilling tool as defined in Claim 1wherein said tool joint housing comprises a two part sleeve with anupper section having a threaded portion adapted to be secured to the endof a drill pipe string and a lower sleeve section secured to said uppersection in axial alignment therewith and having internal splines thereinslidably and drivingly engageable with external splines on said tungstencarbide drag bit.
 3. A drilling tool as defined in claim 1 wherein saidtungsten carbide drag bit and said diamond drill bit include coolantflow passages therethrough.
 4. A drilling tool comprising, incombination, a tubular tool joint housing adapted to be secured to theend of a drill pipe string, a first drill bit mounted within said tooljoint housing in driving engagement therewith, a second drill bitmounted within said tool joint housing in driving engagement with saidfirst drill bit, and means operatively interconnecting said tool jointhousing, said first drill bit and said second drill bit for extendingsaid first drill bit into drilling position and simultaneouslyretracting said second drill bit in response to a downward thrust onsaid tool joint housing by said drill pipe string in excess of apredetermined thrust force, and for extending said second drill bit intodrilling position and positively retracting said first drill bit inresponse to a downward thrust on said tool joint housing by said drillpipe string less than said predetermined thrust force.
 5. A drillingtool comprising, in combination, a tool joint housing adapted to besecured to the end of a drill pipe string, a first drill bit mounted insaid housing for axial telescoping movement with respect thereto andoperatively engaged for rotary driving movement by said housing, asecond drill bit mounted within said first drill bit for axialtelescoping movement with respect thereto and operatively coupled withsaid first drill bit for rotary driving movement thereby, compressiblyresilient means interposed between said first drill bit and said seconddrill bit for biasing said second drill bit axially outwardly of saidfirst drill bit, and means interconnecting said tool joint housing, saidfirst drill bit and said second drill bit for moving said second bit andsaid first bit axially with respect to each other and position saidfirst bit in drilling position in opposition to said biasing means uponthe exertion of a downward thrust on said tool joint housing in excessof a force required to compress said biasing means, and for releasingsaid biasing means to urge said second bit axially outwardly from saidfirst bit upon the exertion of a downward thrust on said tool jointhousing less than the force exerted by said biasing means.
 6. A drillingtool as defined in claim 5 wherein said biasing means comprises a stackof compressed Belleville spring washers.
 7. A drilling tool as definedin claim 5 wherein saId interconnecting means comprises a plurality ofsheaves mounted on said first drill bit and a corresponding number ofcables secured at their ends to said tool joint housing and said seconddrill bit respectively and passing over said sheaves.
 8. A drilling toolas defined in claim 5 wherein said interconnecting means comprises aplurality of gears secured to said first drill bit and racks on saidtool joint housing and said drill bit meshing with said gears atdiametrically opposed points thereon so that said second drill bit movesaxially oppositely to said tool joint housing.
 9. A drilling tool forselectively drilling relatively harder and softer rock materials inresponse to a change in the downward drilling force applied to saidtool, said tool comprising, in combination, a tool joint adapted to besecured to the end of a drill pipe and including a driving sleeve, afirst drill bit telescopingly mounted within said driving sleeve forlimited axial movement with respect thereto, cooperatively engagingmeans on said driving sleeve and said first drill bit for providing asliding and rotary driving connection therebetween, said first drill bithaving an axial bore therein, a second drill bit having a shafttelescopingly mounted within said first bit bore for limited axialmovement with respect thereto, cooperatively engaging means on saidfirst and second drill bits for providing a sliding and rotary drivingconnection therebetween, means acting between said first drill bit andsaid second drill bit for biasing said bits axially apart, and meansoperatively coupling said first drill bit, said second drill bit andsaid tool joint sleeve for moving said second drill bit and said firstdrill bit axially with respect to each other and position said firstdrill bit in drilling position in opposition to said biasing means uponthe exertion of a downward force on said tool joint sleeve greater thanthe force exerted by said biasing means, and for releasing said biasingmeans to urge said second drill bit axially outwardly from said firstdrill bit into drilling position upon the exertion of a downward forceon said sleeve less than the force exerted by said biasing means wherebysaid drill bits can be selectively positioned into drilling position.10. A drilling tool as defined in claim 9 wherein said biasing meanscomprises a stack of Belleville spring washers.
 11. A drilling tool asdefined in claim 9 wherein the means operatively coupling said firstbit, said second bit and said tool joint sleeve comprises a plurality ofsheaves on said first bit and a plurality of cables corresponding innumber to said sheaves connected at one end to said tool joint sleeveand passing over said sheaves and having the other end connected to saidsecond bit whereby an increase in thrust on said tool joint sleeve inexcess of the thrust determined by said biasing means retracts saidsecond bit relative to said first bit and extends said first bit intodrilling position.
 12. A drilling tool adapted to be secured to the endof a drill pipe for selectively drilling both soft and hard rockmaterials comprising a tool housing, a tungsten carbide drag bitincluding a tubular body telescopingly and drivingly mounted within saidhousing, a diamond drill bit telescopingly mounted within said tungstencarbide drag bit body and rotatively drivingly engaged with saidtungsten carbide drag bit, means operatively interposed between saidtungsten carbide drag bit and said diamond drill bit for biasing saiddiamond drill bit into a forwardly extended drilling position relativeto said tungsten carbide drag bit, and means operatively interconnectingsaid tungsten carbide drag bit, said diamond drill bit and said toolhousing for retracting said diamond drill bit relative to said tungstencarbide drag bit in opposition to said biasing means in response to anapplied drilling force transmitted to said tungsten carbide drag bit bythe drill pipe which exceeds the force of said biasing means andPositions said tungsten carbide drag bit in drilling position, saidbiasing means extending said diamond drill bit into drilling position inresponse to a reduction in drilling force applied to said drill pipebelow the force exerted by said biasing means whereby said tungstencarbide drag bit can be selectively positioned under an increaseddrilling force for drilling relatively soft materials and said diamonddrill bit is positioned in drilling position in response to a reductionin drilling force for drilling relatively hard materials.
 13. A drillingtool including a tungsten carbide drag bit adapted to drill relativelysofter materials upon application of a relatively larger downwarddrilling force, and a diamond drill bit adapted to drill relativelyharder materials upon the application of a relatively lesser drillingforce, said tool comprising, in combination: a tool joint adapted to besecured to the end of a drill pipe; a driving sleeve secured to saidjoint; a tungsten carbide drag bit head telescopingly mounted withinsaid driving sleeve for limited axial movement with respect thereto andhaving an axial bore extending therethrough; spline means on saiddriving sleeve and said head for providing a rotary driving connectiontherebetween; tungsten carbide drill bit inserts secured to the drillingend of said head; a diamond drill bit shaft telescopingly mounted withinsaid drag bit head bore for limited axial movement with respect to saidhead; said diamond drill bit secured to the outer drilling end of saidshaft; means on said diamond drill bit slidably engaging said tungstencarbide bit head for providing a rotary driving connection therebetween;spring means acting between said drag bit head and said diamond drillbit shaft for urging said head and said shaft axially apart; and meansoperatively coupling said shaft, said head and said sleeve for movingsaid head and said shaft axially with respect to each other inopposition to the force of said spring means upon the exertion of adownward force on said sleeve greater than said spring force, and forreleasing said spring to urge said shaft axially outwardly from saidhead upon the exertion of a downward force on said sleeve less than saidspring force; whereby said tungsten carbide drag bit is positioned todrill relatively softer materials upon the application of a relativelylarger downward drilling force on the drill pipe and said diamond drillbit is positioned to drill relatively harder materials upon theapplication of a relatively lesser drilling force on said drill pipe.14. A drilling tool including a tungsten carbide drag bit and a diamonddrill bit comprising, in combination: a housing generally tubular inconfiguration and adapted to be secured at one end to a drill pipe; adriving sleeve secured at one end to said housing and having internalflutes at its other end; a tungsten carbide drag bit head telescopinglymounted within said driving sleeve and having external flutes slidinglyand drivingly engaged with said driving sleeve flutes; said drag bithead having an axially extending bore therethrough and an internalshoulder defined in said bore adjacent the outer end of said drag bithead; tungsten carbide drill bit inserts secured to the outer drillingend of said drag bit head; a diamond drill bit shaft telescopinglymounted within said drag bit head; a diamond drill bit head secured tothe outer end of said diamond drill bit shaft and drivingly engaged withsaid drag bit head; an external flange on said diamond drill bit shaftdefining a shoulder thereon, said flange being engagable with saidinternal shoulder on said drag bit head for limiting axial movement ofsaid shaft with respect to said drag bit head; means defining aninternal shoulder adjacent the rear end of said drag bit head; springmeans intermediate said diamond bit shaft and said drag bit for urgingsaid shaft forwardly and outwardly oF said drag bit; and meansoperatively connecting said shaft, drag bit and driving sleeve adjacentthe tool joint ends thereof for retracting said shaft against the forceof said spring upon movement of said driving sleeve forwardly withrespect to said drag bit head to position said tungsten carbide bit indrilling position upon an increase in force upon the drill pipe, and forretracting said tungsten carbide drag bit head and enabling said springto extend said diamond drill bit shaft to position said diamond drillbit in drilling position and simultaneously retract said tungstencarbide drag bit head upon a relaxation of pressure upon said drillpipe.